Download Section 4.2

Objectives
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Describe the structure of an atom.
Explain how isotopes can be used to study biological processes.
Explain the role of an element's electrons in determining its chemical reactivity.
Key Terms
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atom
proton
electron
neutron
nucleus
atomic number
isotope
radioactive isotope
Different elements have different properties. Some are solid metals at room temperature.
Some are invisible gases. Some elements readily react with other elements, while others
hardly react at all. These properties affect the roles that different elements play in
biological processes. This section describes how an element's properties are related to its
structure.
Atoms
Each element consists of a single kind of atom that is different from the atoms of all other
elements. An atom, which gets its name from the Greek word atomos meaning
"indivisible," is the smallest possible particle of an element. In other words, a carbon
atom is the smallest possible "piece" of the element carbon. And that's a very small
piece—it would take more than three million carbon atoms to stretch across the period
printed at the end of this sentence.
Atoms of all elements are made up of even smaller components called subatomic
particles. A proton is a subatomic particle with a single unit of positive electrical charge
(+). An electron is a subatomic particle with a single unit of negative electrical charge (-).
A third type of subatomic particle, the neutron, is electrically neutral, meaning it has no
electrical charge. An element's physical and chemical properties depend on the number
and arrangement of its subatomic particles. For example, the shiny luster of copper metal
and the boxy crystals of sulfur are based on the structure and interactions of the atoms
that make up those elements.
An atom's protons and neutrons are tightly packed together, forming a central core called
the nucleus. Electrons, which have much less mass than neutrons and protons, continually
move about the outside of the nucleus at great speed. The attraction between the
negatively charged electrons and the positively charged protons keeps the electrons close
to the nucleus.
Notice that the model of the helium atom in Figure 4-4 has 2 protons and 2 neutrons in its
nucleus. This model, which is not drawn to scale, represents the moving electrons as a
spherical "cloud" of negative charge surrounding the nucleus. Because the exact path of
any electron cannot be determined, the cloud model is helpful. An electron may visit
every point around a nucleus over time. Thus you can think of the electron's negative
charge as spread out, like a cloud, in all the places the electron might be. In a real atom,
the electron cloud is much larger than the nucleus. To give you an idea of the difference,
consider that if the electron cloud of an atom were big enough to fill a baseball stadium,
the nucleus would be only the size of a housefly on the field!
Figure 4-4
This model of a helium atom indicates the
number of each kind of subatomic particle it
contains. Though no visual model can
accurately show an atom's structure, models
can help you in understanding certain
aspects of an element's chemical behavior.
An important difference among elements is the number of protons in their atoms. All
atoms of a particular element have the same number of protons, known as the element's
atomic number. Thus, a helium atom, with 2 protons, has an atomic number of 2. Left
alone, an atom tends to hold as many electrons as protons. In that state, the atom is
electrically neutral—the positive charges on the protons exactly balance the negative
charges on the electrons. However, the number of electrons is not constant like the
number of protons. Certain atoms can lose one or more electrons, while some atoms can
gain one or two electrons. As you'll see later, the number of electrons determines how the
atom interacts with other atoms. Indirectly then—by setting the usual number of
electrons—the number of protons determines the atom's properties. No two elements
have the same atomic number (proton number), so no two elements have the exact same
chemical behavior.
Isotopes
Some elements have alternate forms called isotopes. Isotopes of an element have the
same number of protons in their atoms but different numbers of neutrons. Figure 4-5
shows the numbers of subatomic particles in atoms of the three isotopes of carbon.
Carbon-12 (usually written 12C), which has atoms containing 6 neutrons, makes up about
99 percent of all naturally occurring carbon. Most of the other 1 percent is carbon-13
(13C), which has atoms with 7 neutrons. A third isotope, carbon-14 (14C), has atoms with 8
neutrons and is very rare. Notice that atoms of all three carbon isotopes still have 6
protons—otherwise, they would not be carbon. Both 12C and 13C are stable isotopes,
meaning their nuclei do not change with time. The isotope 14C, on the other hand, is
unstable, or radioactive. A radioactive isotope is one in which the nucleus decays (breaks
down) over time, giving off radiation in the form of matter and energy.
Figure 4-5
Atoms of three isotopes of carbon differ only in their numbers of neutrons. The isotopes are
named for the total number of particles in their nuclei (protons plus neutrons). Carbon-13,
for example, has 6 protons and 7 neutrons, for a total of 13.
Radioactive isotopes have many uses in research and medicine. Living cells use
radioactive isotopes just as they would use the nonradioactive forms. Thus, radioactive
isotopes are useful as "biological spies" for observing what happens to different atoms
within organisms. Scientists can track the presence of radioactive isotopes with
instruments that detect radioactive decay. Though radioactive isotopes have many
beneficial uses, the particles and energy they give off can also damage cells. However,
the doses of most isotopes used in medical diagnosis, such as a body scan, are relatively
safe.
Electrons and Reactivity
How does an atom's structure determine how it reacts with other atoms? The key is the
atom's electrons. Electrons differ in the amount of energy they have and how tightly they
are held by the protons in the nucleus. Based on these properties, chemists describe an
atom's electrons as belonging to certain energy levels. Usually it is the electrons in the
highest energy level of an atom that determine how that atom reacts.
The first, or lowest, energy level (nearest the nucleus) can hold 2 electrons, while the
second energy level can hold 8 electrons. For example, a hydrogen atom has 1 electron.
Since electrons fill the lowest energy level first, hydrogen's electron occupies its first
energy level (Figure 4-7).
Figure 4-7
An atom's lowest (first) energy level can hold up to 2 electrons. The second level can hold
up to 8. Notice that the second energy levels of carbon, nitrogen, and oxygen atoms are
unfilled with 4, 5, and 6 electrons, respectively. (Remember that atomic models are limited in
what they can represent. Energy levels are not actual physical locations.)
Helium (modeled in Figure 4-4) has 2 electrons, filling its lowest energy level. Carbon
has 2 electrons in its lowest energy level, and 4 more electrons in its second level. Note
that both hydrogen and carbon have a partly-filled energy level, as do nitrogen and
oxygen. That condition makes these atoms chemically reactive—they tend to react with
other atoms, filling their highest occupied energy levels. In contrast, a helium atom,
which has no partly-filled energy levels, is inert—it does not tend to react. In the next
section, you'll read more about the ways electrons are involved in the reactions among
atoms.
Concept Check 4.2
1. Describe three kinds of subatomic particles and tell how they are arranged in an atom.
2. What is an isotope? Explain how radioactive isotopes are useful to researchers.
3. Describe the significance of the number of electrons in an atom's highest energy level.
4. Explain the significance of an element's atomic number.
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